REPORT No. 272
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REPORT No. 272
THE RELATIVE PERFORMANCE OBTAINED WITH SEVERAL METHODS OF CONTROL . OF AN OVERCOIVH?RESSED ENGINE USING GASOLINE
By ARTHUR TV. GARDIXER and W3EDON Langky Memorial AerctnauficaI Laboratory
327 . REPORT No. 2’72
THE RELATIVE PERK)RAI.AN!E OBTAINED WPMI SEVERAL METHODS OF CONTROL OF AN OVERCONIPRESSED ENGENE USING GASOLINE
By ARTETR W. GARDIX-ER ancI WmLIAM E. W’H~DON
SUMMARY
This report presents some results obtained at the Langley Xemo~”al A.wonautical -lkbratory OJ the National Aiil%ory Cornmdtee for Aeronautics, during an investigation to determine the relatire perf ormanee characteristics for serei-al methods of control of an orercompressed engine using gasoline and operating under sea-lerel conditions. For this work, a special single cylinder test engine, /i-inch bore by Y-inch stroke, and designed for ready adjustment of compression ratio, valze timing am? cahe @ while running, was used. TAis engine has been fully described in N. A. ~- A. Technical Report No, 250. Tests were made at an engine speed of i ,400 B. P. X. for compression ratios ranging from .&O to T.6. The airduel ratios were on the rich side of the eherfiicaZZycorrect mixture and were approximately those giring maximum power. Men using plain domestic ariation gasoline, de- tonation was contro~ed to a constant, prede~ermined amount (audible), such as would be permissible for continuous operation, by (a) throttling the carbure~or, (6) maintaining fuZ throttle but greatly retarding the ignition, and (c) varying the timing of the inlet calce to reduce the e~eciire compre%- 5-ion ratio. For the first and third methods, the throttle opening and the oalce timing, respect irely, were adjusted so Llat tiie ignition timing cmdd be adranced slightly beyond the adcance gim”ng maxi- iizum power without ezceeding the standard of permissible detonation. The optimum performance for the engine when using a nondetonatingfuel, consisting of 80 per cent of commercial benzol and 20 per cent of aviation gasoline, was obtained as a ksisfor comparison. .— The following comparative results are based on the optimum performancefor the engine obtained with the nondetonating fuel at a compression ratio of .47. The poxer and f uel consumption with method (6) remained substantially constant at the higher compression ratios, the order of the ignition timing permitting full throttle operation ranging from SOOat 4.?’ to 3“ at 7.8; ezhaud temperatures, heat loss to the cooling water and explo~ion pressures at the higher ra~ios were normal. At a com- pression ratio of 7./7,the power obtained m“th method (a] was about 39 per cent lew and the fuel conwmption zoas considerably lover; uith method (b), time of inlet-rahe opening constant and &ime of in7et-Uake closing varied, the power was about 23 per cent less and t~e fuel co nsumpfion was greatly increased; with method (c), time of inlet opening and closing varied simultaneously, the power was about 29 per cent less and ~hefuel consumption was greatly increased. From these results, it may be concluded that method (b) gires the best all-round performance and, being easily employed in service, appears to he the most practicable ‘method for controlling an orercompressed engine using gasoline at 10w altitudes.
INTRODUCTION
Owing to the inherent advantages of higher specific output and reduced fuel consumption resulting therefrom, considerable attention has been directed toward increasing the expansion ratio of the earbureted engine, partiwlarIy the engines used in aircraft-, as it is in this field that the abo~e advantages are in greatest demand. However, coincidentally with an increase in the expansion ratio, there is, in conventional engines, a corresponding increase in the compres- sion ratio wit h comeq uent aggravation of those conditions within the enatie inducing det om+ 9iXJ7-2&22 329 330 REPORJ? NATIONAL ADVISORY COMMITTEE FOR AEROX.4UTICS tion and preignition. For this reason, compression r@ios have been limited by the detonation characteristics of the fuels available for genera~ service use and, where higher compressions have been employed, it has been necessary to resort to the use of special fuels, which have no~ been, and are not now, at once cheap and generally available. Thus, the empkyment of higher compression ratios in aircraft= engines has not been generally adopted. However, as conditions inducing detonation hi the high-compression engine become less severe at high altitudes, a compromise has been sought, in which relatively high-compression ratios are employed in conjunction with throttling at low aItitudes to reduce the density of the charge and fihus suppress detonation until sufficient altitude has been gained to permit full-throttle operation, This scheme has received approval because it permits the use of fuels that are generally available, resuIts in increased fuel economy (greater expansiou ratio) at all altitudes, and gives an altitude range of constant power output. For operation below the altitude permitting full-throttle operation, the maximum permissible output; -eomparcd to the maximum obtainable with the same engine using a nondetonating fuel, is considerably reduced, and the specific weight of the engine, even when de~gned especially for the reduced stresses accompanying the reduced power, is increased. Thus, the questions arise: How much throt- tling is required to suppress detonation at low- altitud~s., does the performance under this condi- tion compare favorably with that of a no~mal engine., permitting full-throttIe operation \vitJl the common fuel} and is this method of control, by throttling, the most advantageous? In this connection, Ricardo has shown (Reference 1) that t.be specific output of an overcompressed engine throttled to prevent detonation is relatively low. But information showing the rela~ive performance obtained with other methods of controI is. apparently, not available. The present investigation was undertaken, therefore, at the Langley Memorial Aero- nautical Laboratory of the National Advisory Committee for Aeronautics iu connection with ,a research program invoking the determination of the &Qmparative flight performance of normal, overcompressed, and supercharged engines. The immediate object was to determine ~he rela- tive performance of a single-cylinder, four-stroke-cycle, high-compression engine using domestic aviation gasoline with detonation controlled to a wnstant, predetermined, and permissible amount by throttling, retarding the ignition timing and \-arying the inlet valve timing. The full-throttle performance for optimum conditions when using a nondetonating fuel consisting of 80 per cent of commercial benzol and 20 per cent aviation gasoline was also obtained fc)r comparison, The range of compression ratios _used was from 4.0 to 7.6. The tests herein reported were conducted under.. the immediate. supervision of C!ljde R. Paton. DESCRIPTIO&_
For the present investigation a special, single~c.ylinder, four-stroke-cycle test engine, 5-inch bore and 7-inch stroke, connected to a 40/100 HP. electric cradle dynamometer was used. This engine, a transverse cross section of which is showu.in Figure 1, has been fully described in Reference 2. Briefly, the following adjustments can be made while the engine is running: A change in compression ratio from.4 to 14; a variatio~ in the time of opining and closing of both the inlet md exhaust vaIves of 50° measured in terms of crank tra-rel, the opening and cIosing being controlled independently, and a variation in the lift of both the inlet and exhaust valves from ~ inch tO M inch. Overhead val~~es are used, the two i-rdet and tht two eskmst. valves being operated by independent cam shafts. The water jackets for the cykder head and cylinder barre~ are separate, so that the temperature of the Cooling water for each may be controlled independently. For the present tests the compression ratio was varied from 4-to 7.5, and for all tests except those involving a varied timing the following valve adjustments were used: Inlet open ...... ______JJl_degrees after top center. Inlet closed ------AL5degrees after bottom center. Inlet-valve lift ------.* inch. Exhaust open------JiOdegrees before bottom center. Exhaust-closed ------M) degrees after top center. Exhaust-valve lift ------.~ inch. METHODS OF CONTROL OF AN OTEP.C!OMPRESSED EXGIXE T7SIIN”G GASOLINE 331
FIG I.—Transrersesection throughsingIe-cyIiridertestengine,aho%ng adjustmentfor vrwing the compression ratiowhiIerunning 332 REPORT NATIONAL ADT71SORY COMMITTEE FOR AERONAUTICS
For the tests with constami inlet-valve opening and Iate closing, the latter was varied from 45° to 108° after bottom center. For the tests with the total irdet phase displaced in the cycle, the phase was maintained at 125° and the time of opening varied from 10° to 60° after top center. Two porcelain-insulated spark plugs located diametrically opposite on the longitudinal axis of the cylinder were used; these were timed synchronously. A standard Liberty engine duplex carburetor was used, one Venturi being completely closed and the main metering jet for the remaining Venturi being provided with a needle -vaIve for ready adjustment of the fuel flow. Some tests were made with a small fixed metering jet, the needle ~,a]ve being omitted. FueI consumption was measured by timing the rate of flow from a 400 cm.3 tank Domestic aviation gasoline, conforming to Army spcwifications, was used as the detonating fuel. A mixture of 80 per cent of benzol (commercial 90 per cent) and 20 per cent a~iation gasoline (80–20 blend) was used as the nondetonating fueI. A diagrammatic layout of the induction system is shown on Figure 2. Air measurements were made with a gasometer having a displacement volume of 10 cubic feet; the gasometer belI
1
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The tests herein reported -were made at a constant ergine speed of 1,400 R. l?. M., aIthough, in some cases, additio~al tests were made at 1,200 and 1,600 R. P. M. to check the generaI trend of the data obtained at the intermediate speed. For any gi~en test, after conditions were . stabilized, a po-wer run -was made extending over a time inter-ral a-rera=ging from two to three minutes, during ~hich readings were taken of engine speed, torque scaIe, rate of fuel and air consumption, exhaust temperatures, and expIosion pressures, and the -water from both the cylinder head and cylinder barrel di-r-erted into containers and weighed for the determination of heat dissipation to the cooIing -water. Immediately folIowing each power run, and for the same engine settings used during the power run, the circtiation of cooling water was shut off and a friction run was made by motoring the engine -with the dynamometer. Compression pressures were measured during these friction runs. The testing methods employed with the se~-eral schemes of operation are thought to be of sufficient interest to merit a detailed description: 1. Effect of ignition advance on the performance of a high-compression engine when using (a) a detonating fuel and (b) a nondetonating fue~ (fig. 3): In this case, the compression ratio was fked arbitrarily at. 6.3, the carburetor needle-valve was maintained at a constant set tiq and full-throttle performance obtained at progressi~ely increasing ignition ad-rances ranging from top center to 50” ad-rance. 2. Optimum performance with a nondetonating fuel: FuI1-throttle. performance -was obtained for the range of compression ratios from 4 to 7.3. At each compression ratio, the ignition setting and air-fuel ratio were adjusted to give ma-ximum pom-er. 3. Throttled performance using a detonating fuel: (a) ‘With a fixed ignition advance: In these tests, the ignition ad-rance was ti~ed arbitrarily, the air-fue~ ratio was adjusted for each run to an approximately constant value of 12.2. Full-throttle runs were made at low compression ratios, and, as the compression ratio was increased, the throttk was adjusted to give the standard of detonation. @) Throt tied performance with norm a~ optimum ignition adv-ante: In these tests, the arbitrary criterion for the permissible amount of throttle opening selected -was the greatesb opening that permitted the ignitioa to be adwmced slightly beyond the advance giving maximum power without. exceeding the standard of detonatiort. At a gi~en compression ratio, the throttle opening was fixed arbitrarily and severa~ preliminary runs made at increasing ignitiofi ad~ances until the standard of detona- s tion was obtained; the throttle was reset at a lesser opening and preliminary runs made as above, until the arbitrary condition had been fulfilled, following which a run was taken for this throttle opening and the optimum ignition advance. Tests for compression ratios from 4.5 to 7.5 were made with a fixed carburetor jet. giving an approximate air-fuel ratio of 12.2. 334 .REPORT N.*TIONAL ADVISORY COMMITTEE FOR AERON.4UT1CS
4. Full-tllrottle perforl~lance \vitllgreatlyre tardedig]~itio1l: In these tests, fullthrottlewas maintained and the ignition timingwas retarded sufficiently to give the standard of deton~tion at compression ratios of 5.3, 6.3, and 7.3. Theair-fuelm ixture was adjusted togiveapproxi- rnately maximum power. 5. Full-throttle performance with varied timing for the inlet valve: Tests wwemadewith fixed carburetor metering jets giving an approximate e air-fuel ratio of 12.2. For a given com- pression ratio, the valve timing was set arbitrarily and the ignition adjusted to give the standard of detonation; the valve timing was readjusted and the. ignition advance varied until a valve timing was determined that permitted the i~gnition timing to be advanced slightly ahead of the advance giving maximum power without exceeding the standard of detonation. A run was then made for the valve timing so determined and with the optimum ignition admmce. Two methods of varying the inlet valve timing were used: (a) ~~ith the time of opening fixed, and the time of closing varied; range of com- pression ratios from 6 to 7.5. . (?I) With the time of opening and closing varied simultaneously; range of com- pression ratios from 5.1 to 7.6.
PRECISION
Changes in atmospheric conditions caused relatively large variations in the amount of detonation, as evidenced, for instance, by the fact that, for otherwise constant engine conditions, the amount-of throttle opening permissible without exceeding the sttindard of detonation varied somewhat on successi~e days. This caused sotn e discrepancies in the engine settings nec-essary to fulfill given requirements, and introduced unknown errors in the compfirative results. As these variations affected the results probably to a greater extent than any instrumental or personal errors, a discussion of the latter is thought to be unnecessary. For the above reason, precise comparisons of the relative performances can not be made. However, the data are sufficiently precise to permit, of making some general comparisons.
RESULTS
The results are presented in the form of curves on Figures 3 and 4, the comparative per- formance being presented as plots on a percentage basis for ready comparison. The base values are for the optimum performance of the engine using a nondetonating fuel at. a con~pres- sion ratio of 4.7 to 1; these base values are given on the several curves. Power has been cor- rected to a standard pressure of 29.92 inches mercury. Some comparative results t.akcn from Reference 1 are also shown. DISCUSSION
For the reason that some detonation is permissible, and that-maximum power for a gi~en fuel is usually obtained when a small amount of detonation is present, a criterion of “some” detonation rather than “no perceptive” detonation was selected. Moreover, many service engines are operated under such conditions; that is, with a certain pernlissib~e amount of detona- tion present during full-throttle operation. The amouni of detonation selected as perinissible in the present tests had no perceptible effect on power or heat losses to the cooling water, and was such as would be permissible in an engine for continuous operation. l?reliminary tests with gasoline at a compression ratio of 7.3 to 1 showed that, for a given ignition advance, the amount of throttling required to l~~it detonation to the permissible ~mount was dependent to a great extent on the quality of the fuel mixture. In one test at a compression ratio of 7.3, increasing the air-fuel ratio from 14.7 to 17.4, with a fixed ignition advance ~f 30°, permitted a greater throttle opening and resulted in an increase in power of more than 14 pcr cent. On the lean side, the mixture permitting the greatest throttle opening, and consequently giving maximum power, varied considerably with a change in ignition advance; for one t~wt, ab a compression ratio of 7.3, the air-fuel ratio giving maximum power changed from 15.8 to 18,7 for a change in ignition adavnce from 5° to 20°, The range of air-fuel ratios giving maximum power METHODS OF CONTROL OF AN OVEKOXIWESSED E>~GINE USING GASQLIKE 335. on the rich side aIso -v=ariedwith the ignition advance but, in genertdj ih was found that these ratios ranged from 11.7 to 12.3 for all compression ratios. For the results presented herein, the air-fueI ratios were approximately those gi-ring maximum power on the rich side, thus simulat ingz approximately, flight, setice conditions. Howe-rer, some ~ariations in these ratios ha~e entered to influence the results, and to pre~ent obtaining precise comparisons. The information given in Figure 3 is presented to indicate the effect of ignition advance on engine performance RLa.comprcsion ratio of 6.3 when operating ab full throttle with plain a-i-ia- tibn gasoline, and with a nondetonating fuel. The carburetor needIe-valve adjustment was slightIy different. for the two fuek, bub was maintained constant for each fuel; these settings
—
lgnh%m udtmnce, degrees beibre %c. Fu:.3.—EEectof knition advanceon enginepsfarm.me at a constantspeed of 1,~011R. P. M. and a eonstarttcompressionratio of 6.3:1when msinggxoline and a nondetonatingfuel. Percentagesareba.~cfon vaiuesobtained with the nondetonatingfuel for optimum power at W“ ignitionadnnce. Thesebasevaluesarenoted on the wreral cnrres. The noteson detonationreferto operationwith gxoIine did noh necessarily gi-re maximum power. There -were wide variations (from 12 to 13.5 for ga~o- line and from 10 to 12.5 for the 80–20 Mend), in the air-fueI ratios, which caused some irregularities in performance. From these data, it is seen that, -when using gasoline, there is a marked increase in the se~erity of detonation as the ignition thing is advanced beyond 10°, resulting tinally, for an advance of 45”, in excessive preignition and fused spark-phg electrodes. Under these severe detonation conditions, there is a marked decrease in power and an increase in the heat losses to the cooling mater. ?Then using the 80-20 blend, there is a normal reduction in power for ignition advances beyond the optimum of 30°, but the heat losses remain fairly constant, or e-i-en decrease sIightly, and the explosion pressures increase uniforndy. Aht.ention is calIed to the fact tha~ maximum power with gasoline was obtained at an advance sIightIy greater than the advance giving the arbitrary standard of detonation; also, that the extreme detonation did not 336 REPORT NATIO>’AL ADVISORY COM2JITTEE FOR AERONAUTICS tend to increase the exhaust temperatures. The e~plosion pressures when using gasoline in- creased rapidly for ignition advances above 20°. Although the maximum-pressure gage was noi reIiable for measurements under these conditions, the indication was that pressures of the order of 1,600 pounds per square inch existed. For the range of ignition timing resulting in severo detonation, there was probably some power lost as negative work on the piston due to incipient preignition. The power runs in this region were mairitainecl only for a period of about two min- utes. From these results, it is apparent that full-throttle operation with gasoline at this com- pression ratio couId not be maintained continuously for ignition advances greater than 9-10°. The optimum performance when using the 80–20 blend and the compamtive performance obtained by the serveral methods of control when using gasoline are shown on Figure 4. The performance curves are numbered in accordance with the tesbconditions enumerated in the title. for this figure. Considering the optimum performance of the engine using the 80–20 blend, it is found that the variation in indicated power and indicated specific fue~ consumption with change in com- pression ratio is approximately that given by the ratio of air-cycle efficiencies. There is an id i- ca Lion that power increases with compression ratio at a faster rate than given by the cycle efficiencies, but this slight discrepancy may be attributed to smalI errors introduced by tha method of obtaining friction power. It is seen that the optimum ignition advance dec~eascs slightly with increase in compression ratio, a decreas? from 37° to .26° being noted for a change in compression ratio from 4 to 7.3. The observed optimum air-fuel ratios for the different compression ratios varied (from 11 to 12.5) more t,hm would be expected, but, as the direction was not consistent and as power was not very sensitive to changes of mixture in this range, an average value of 12 may be considered as representing the optimum for all mtios. In this connection it should be kept in mind that the chemically correct air-fuel ratio for this fueI is less than for gasoIine. The variations In air-fuel ratios have caused some irregularities in the observed values for specific fuel consumption. Tests made with gasoline at a fixed ignition advance and throttling to suppress detonation will next be considered. In this case it was found that; for a fixed advance of 30°, full-throttle operation could be maintained up to a compression ratio of 4.7 without-exceeding the standard of detonation, but that at the higher ratios considerable throttling was necessary, with a resultant sharp decrease in power. Values for power at the higher ratios were obttiined from moss plots, and no data points are shown. For an advance of 24-0full throttle could be maintained up tu a compression ratio of 5, and for 18.5°, Up to 5.3. For these Iower adl’antes the full-throttle power at- the lower compression ratios was reduced, as the timing used was Iess th~n the optimum, but–at the higher ratios the permissible power increased as the timing was retarded. This comparison points clearly to the sensitiveness of ignition timing in its influence on deto~a- tion, and hence to the amount of throtthng necessaxy and the permissible power output at the higher ratios. The above discussion leads logically to a consideration of the results obtained Ivhen the ignition timing was retarded sufficiently to permit full-throttle operation at all compression ratios investigated. From the above discussion it was seen that permissible power increased with decrease in ignition timing at the higher ratios. Thus it would be expected that maximum power at these ratios would be obtained by carrying this procedure to the knit. Suoh was found to be the case, and of those methods investigated retarding the ignition timing to perrniti full-throttle operation proved to be the most advantageous from the standpoint of power. ‘ilTith this method of control, the permissible power remained substantially constant for all ratios from 4.7 to 7.3, whereas for the other methods the power decreased at the higher ratios, It is noted that the power obtained by this method at the intermediate ratios approximates thit obtained by Ricardo (Reference 1, p. 92) when admitt”mg cooled exhaust gases with the intake charge to suppress detonation in an overcompressed engine using fueI having a low tolucne value. (Fig. 4.) Fuel consumption with retarded ignition was not excessi~e, but remained practically constiant at the value of 0.532 pound per” I, HP, per hour, obtained with the non- detonating fueI at a compression ratio 4.7. Heat losses to the cooling water and the exhausi temperatures also remained normaI. The ignition advance found necessary to give these METHODS OF CONTROL OF AN OVEWOMPRESSED ENGINE USING G.\SOLIXE 33”7
1 I I ! ! r 55 ,, !,,!, 1 ! 1 il iti iil ii itiiililil J 4.0 ’43 4.6 4.9. 5.> ‘ 5.5 58 6./ 6.4 67 %0 Z3 %6 Compression rofio
? Fu[-fhffle:8L?Der c-f bei-zzo/-2Oner cenf aviuftbn gasohhe; besf n?i>ue und @nitkw dv~c-e~ 2 Fulk%-off!e;atiftin gasofk; @m?.& ++ reforw&d iotif ddonaf ion foap-edefertied ornormi;QWOXY- mofe/y besf PO wer m&fure. 3 Ful[-fhrofffe;gasake; fineoftilef-valveope-z?q hk~ fimeoftief-valvecfosiq aaj-usfedfopermif ,@nifi&wfti- ,’hgfobe advanced sl[ghf[ybeyand fhe advance @ing “ -- — besf power wifhwf exceeo%gpermiss!~,(e&fona//on. 4 fol/-fh.offie;gosahtie;fineof ktkf-valveopemhg and absiq varied.simufr%eouslyfopermif besf I@i%bn udvonce (see 3. ~ gperafb.’ guso[ti; .&i-oflle Gc7@sfed +o~m? &ifion i%zhgfobe &wGrrced s8gh& beycwd odvonce giv- k?gbesfpaw~ wifhoufexceedhgpermismhle de.fcwu+ti. 6 Throfffsdgaso,kne;ffrrof,+%dfa [ma defanafibn wifh COmpre.ss&47 rdio 77xedignifioni%mi-tg.
Q I I 1111 I I I I I 1 1> 0 Iltlllll 16!41 I;0 I:>&i’ , 40 4.!5 52 58 %6 Compessim rofio Compressiti rafio FIG.4.—Comparativeperformanwobtained with severalmethodsof controlof an o~ercurupres.s+dengine. Performraucesshown as CerCW2tages basedon ~aluesobtainedwith a nondelonatfugfuelat a compressionratio of J.7. BaseMues are shown on the variouscurws. The curres arenumberedto correspondto the condition?of operationasgi~en 338 REPORT NATIONAL .4DVISORY COMMITTEE FOR AERONAUTICS results varied from 30° at 4.7 to 3° Et 7.3 compression ratio. Ignition timing at the higher ratios was very critical, small changes in timing causing considerable variation in the amount of detonation. This is probably the greatest disadvantage of this method for service use. The present- tests do not show to what extent the compression ratio may be increased with this method of control, but apparently this method may be used advantageously up to a compression ratio of at least S :1. The performance obtained when throttling the carbure~or just sufficiently to permi~ th~ ignition timing to be advanced, for each compression ratio, slightly beyond the advance giving maximum power will next be considered. The condition stipulated in these tests is, admittedly, an arbitrary onej but=it fulfills, in generalj the service condition where the ignition advance at low altitudes is maintained constant at the adjustment giving maximum performance for dtitude.s above that permitting full-throttle operation. It may be seen from Figure 4 that the optimum ignition timing for this method of control, and also for the method discussed below in which the inlet-valve timing was varied, approximates, for the various compression ratios, the optimum ignition timing determined for the nondetonating fuel. The power obtained by throttling under these conditions approximates, at the higher ratios, that obtained for the fixed ignition timing of 30°. This would be expected, as the optimum timing determined with
F@. 5.—Inlet-wdvetimingusedforconditions(3) and (4), Figure4
this method varied but slightly from 30°. The permissible power output at a compression ratio of 7,3 is seen to be about 40 per cer+t less than.~btained by maintaining full throttle and retarding the ignition. The indicated specitic fuel cmsumpt,ion is, however, somewhat lo~ver, although the variations in the quality of the fuel mixture have caused a scattering of the data and have thus obscured’ the advantage of the throttling method from this standpoint. It would be expected that the indicated fuel consumption with this method would be but little different from that obtained with the nondetonating fuel. This method would suffer some- what by comparison on a brake basis Olvingto thehigherfrictionlo- with theenginet~lrottlcd. It is worthy of note that hhe percentage decrease in indicated power is less than the percentage decrease in volumetric efficiency at the higher ratio. This method of control, altk.ough giving the best fuel consumption, gives the leasti power. The general trend of the power curves obtained with this method approximates that found by Ricardo (Reference 1, p. 94) when employing the throttling method to give no detonation with a fuel having detonation charac- teristics similar to- gasoline, althought it is not stated in the reference whether a fixed ignition timing or a variable timing was employed. varying tile ~let-valve timing has been employed in England in connection with the use of higher compression ratios in the Bristol Jupiter air-cooled engine. For this reason it seemed pertinent to investigate the relative performance to -be obtained with this method of control. From Figure 4 it is seen that when the time of inlet opening and closing was varied simul- taneously and the ignition maintained in the optimum position rioted in the preceding discus- sion the permissible. power output decreased with ticrease in compression ratio, the decrease at a compression ratio of 7.5 being about 29 per cent. L’~der these conditions there was considerable reversed ffo~v tbrOU@ the in~et ~al~re g@j = a conquen% the fuel consump tiall. XEETHODS OF CONTROL Ol? All’ OVERCOXPRESSED EN-GENE USI1l-G GASOLINE 339 with the single-cylinder engine increased considerab~y as the -raIve timing was ~aried from the nornd. i\Toaccount has been taken of the fuel that was deposited in the air heater as a result. of the reversed air fiow. considerable improvement in fue~ economy would be expected in a muIti-cylinder engine, -where several cylinders are connected to a common manifold. Some improvement in power and fuel consumption -was obtained by maintaining the time of opening constant and varying ordy the time of closing, but the gain is not considerable and the method would not be practicaI for service use. The disadvantage of complexity more than offsefi the ~d-rantage of slightly better power obtained with this method as compared to the throttling method. Figure 5 shows the inlet-~al~e settings empIoyed. For zJI conditions, the heat Ioss to the cooling water in the head remained beIow 10.5 per cent of the total heat in the fuel; and for the cooling water in the cylinder barrel, below 7.5 per cent. When the variable val~e timing was used, these Iosses were considerably reduced ati the higher compression ratios. Exhaust temperatures remained below 1,425° F’. for all conditions of operation above 4.7 compression ratio. For the variable -ral-re-timing method, these temperatures were Likewise reduced considerably at the higher compression ratio. A detailed discussion of the ~~derlying conditions influencing detonation in these tests has been purposely omitted, as the data are not sufficiently precise to permit of making a detailed analysis. CONCLUSIONS
l?rom these comparative tests, it may be concluded that, of those methods in~estigated for controlling an overcompressed en=gine using gasobe under sea-Ievel conditions, m~~imum power is obtained by maintaining full thrott~e and greatly retarding the ignition timing. Also, as the fuel consumption, exhaust temperatures, and heat loss to the cooling water are normaI, and as this method of control is easily accomplished without adding to the complexity of the power p[ant, it may be considered the most practicable method for service use. Throttling the carburetor with, approximately, fuII normal ignition advance gives the best economy of the various methods bu~ the Ieast power. Varying the timtig of the Met valve with, approximately full normal ignition advance gi-ves somervhat ~greater power than obtained by the throttling methodj but the fueI consumption is excessive, although the -ralues for the Iatter gi-ren in this report. are higher than would be obtained with a multi-cylinder eng~e.
L.AXGLEY kfEMoRIAL ~EROXAUTICAL L.LBORATORY, A~ATIOXTAL ADVISORY COMMITTEE FOR AERONAUTICS, LANGLEY FIELD, VA., Fehary .25, 1927.
REFERENCES AXD BIBLIOGRAPHY
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